The set data type has the simplest, stupidest implementation possible: an unordered array. This is because of how JavaScript's data types work. If it were possible to compute a hash value from any data type, or get its memory address, then we could do something more elaborate. If < and > operations were defined for all data types, we could use some kind of balanced tree representation, or sorted arrays. If JavaScript objects supported arbitrary data types as indices, this would all be too easy. But none of those things is true, so we're stuck relying only on the === operation, and unsorted arrays. For small sets, this is not a problem. For larger sets, if performance of set operations turns out to be problematic, you may want to use a specialized set data type. For example, if your set members are all strings, you could represent sets as objects with set members as keys, and it would be fast. For this, use the StringSet class, described below.

Set API

The Set class has the following methods:

new Set(items): Creates a new set. If an array items is given, its contents will be added to the set.

has(x): Does this set contain an element x? Returns true or false.

add(x): Add an element x to this set, and return this set.

remove(x): Remove an element x from this set, if it is part of the set. If it is not part of the set, do nothing. Returns this set.

union(other): Return a new set containing the items found in either this set, the other set, or both.

intersection(other): Return a new set containing the items found in both this set and the other set.

difference(other): Return a new set containing the items in this set that are not in the other set.

symmetric_difference(other): Return a new set containing the items in either this set or the other set, but not both.

issubset(other): Return true if every element of this set is in the other set.

issuperset(other): Return true if every element of the other is in this set.

equals(other): Return true if this set equals another set, i.e. if every element in each set is equal to an element in the other set.

array(): Return a copy of the items in the set, as an array.

size(): Return the size of the set.

copy(): Return a shallow copy of the set.

pop(): Remove and return a random element of the set, or null if the set is empty.

pick(): Return a random element of the set, or null if the set is empty. Unlike pop, does not remove the element from the set.

each(callback): Call a callback function on each element of the set. If the set is changed by the callback, the results are undefined. The callback takes a single argument: the set element that it's being called on.

The condition for determining whether two values are equal is the === operator. Therefore sets can support any mix of data types, as long as the data types can be compared for equality in some meaningful sense with ===.

Specialized sets

If all of your set members have unique string representations, then you can create a set using object properties to keep track of the members. This takes advantage of the fast built-in object type in JavaScript, and is generally better than using general-purpose sets if there will not be collisions, e.g. a set containing both the number 42 and the string "42".

The StringSet class behaves just like the Set class, but it uses this object-based encoding and requires that all set members have unique string representations. Instantiate it with new StringSet(items), and the API is the same as described above.

Note that Set will generally be faster and more memory-efficient than StringSet for sets with fewer than around 110 elements. StringSet is good for large sets, though.